A sensitive method for light microscopic localization of brain receptors by in vitro autoradiography was developed previously in this laboratory. By this method we have mapped the locations of drug and neurotransmitter receptors in the brains of rats and other vertebrates, including primates. Immunohistochemistry is used to compare the locations of transmitter-specific pathways with the locations of the relevant receptors. The non- correspondence, or "mismatch" between the locations of receptors and transmitters in the opiate and tachykinin systems, as well as others, allows hypotheses about parasynaptic circuitry in the central nervous system. Physiological activation of vasopressin dynorphin pathways in the posterior pituitary altered kappa opiate receptor binding in the neurohypophysis, a site of parasynaptic communication. Current and planned work uses similar strategies to better understand the forms of functional intercellular communication.